The present invention relates to a process for producing esters for use as a diesel fuel substitute using a nonalkaline catalyst.
For years, oil and fat feedstocks, particularly waste feedstocks, have been regarded as an important potential source of alternative fuels for diesel engines. Specific feedstocks such as vegetable oils and animal fats have been targeted as desirable sources of fuel because these feedstocks are produced from renewable resources, are biodegradable, and have good emission characteristics. Further, these oil and fat feedstocks, when treated, have gained in commercial importance in a production of fatty alcohols and other oleo chemical products.
Vegetable oil and animal fat-based diesel fuel substitutes have been unsatisfactory, however, because of a high and variable viscosity, variable specific gravity and impurities in the fat and oil. High and variable viscosity and variable specific gravity have prevented suitable atomization of the fat and oil prior to combustion. Additionally, a high viscosity in fat and oil has produced carbon deposition and sticking rings in engines burning the diesel fuel substitutes. It has been found, however, that transesterification of oil and fat reduces viscosity and improves atomization.
Transesterification is a reaction, primarily of triglycerides in fats and oils with an alcohol such as methanol to make esters, such as methyl esters, and glycerine. Transesterification of a mixture of feedstocks obtained from a variety of processes has, however, been problematic. The variety of feedstocks has included hydrogenated and unhydrogenated animal fats, vegetable oils, and waste oil streams. The free fatty acid concentration in these feedstocks has varied widely in concentration from about 0.2% to 50%. The fatty acid types in these feedstocks have also varied and have included monoglycerides and diglycerides.
The feedstocks additionally have included impurities, such as polypeptides and phospholipids, that have not only been difficult to remove in ester production processes but have interfered with the transesterification reaction. The impurities have also interfered with separation and purification of transesterification products. Because of the great disparity in free fatty acid type and concentration, types of impurities and concentration of impurities, it has not been possible to use a single step process to treat a variety of feedstocks to make diesel fuel.
Historically, triglycerides in fats and oils have been methylated or otherwise esterified in a two-step process using an acidic catalyst, such as is described in the Stern et al. U.S. Pat. No 4,695,411, issued Sep. 22, 1987, the Jeromin et al. U.S. Pat. No. 4,698,186, issued Oct. 6, 1987, and the Kawahara et al. U.S. Pat. No. 4,164,506, issued Aug. 14, 1979. Acid catalysts described have included homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts have included concentrated sulfuric acid, KHSO.sub.4 paratoluene sulfonic acid, methane sulfonic acid and so on.
The amount of acidic heterogeneous catalyst has varied from about 1% to 25% depending upon free triglyceride content of the oil or fat. The acidic heterogeneous catalysts include strongly acidic sulfonated ion exchange resins such as Amberlite.TM. manufactured by Rohm and Haas Co., of Philadelphia, Pa., Permutit.TM. of the Permutit Company of Warren, N.J., Dowex.TM. manufactured by Dow Chemical Co. of Midland, Mich. and Lewalit.TM. from Bayer AG of Leverkusen, Germany. An acidic Zeolite Catalyst obtained from SUD Chemie of Glarus, Switzerland, has also been used with some success.
In a first step of the two-step process, the oil and fat feedstock to be transesterified is blended with an alcohol such as methanol. The acidic catalyst is also added to this blend. The blend is then heated and cooled.
In a second step, an alkaline catalyst such as sodium methoxide or KOH dissolved in methanol is added to the oil with additional anhydrous methanol to carry out the transesterification reaction. The temperature of the reaction varies but it is conventional to carry out the reaction between 75.degree. and 80.degree. C.
The two-step process is required because a feedstock having a free fatty acid concentration, when exposed to an alkaline catalyst, produces a high concentration of soap. The soap emulsifies and solubilizes other fat and oil materials in the feedstock, thereby promoting dissolution of these materials in a glycerol layer. Consequently, a preesterification step, is necessary to remove the soap in order to perform the subsequent esterification reaction with an alkaline catalyst such as methoxide.
A transesterification-methylation reaction based upon an ion-alkaline catalyst using a continuous reaction at about 50.+-. bars and about 200.degree..+-..degree.C. has been described by S.A. Extraction DeSmet N.V. of Belgium, in a publication entitled "Fatty Acid Methyl Esters Production by Transesterification." This publication describes a feedstock having a free fatty acid concentration of not more than about 4% free fatty acids.
Ethyl esters have been conventionally produced from triglycerides in an oil or fat feedstock with oils or fats having to esters by reacting the fats or oils with absolute ethanol, in a presence of 2% concentrated sulfuric acid. Thus, an oil such as spent frying oil having an acid value of 11.0 may be converted to a mixture of ethyl esters by reaction with absolute ethanol, for a period of about eight hours. The ethyl esters so obtained have an acid value of about 0.2 and a viscosity of 5.9 centistokes, cSt, at 100.degree. F. Oils having an acid value of between 96 to 100 are converted to ethyl esters to an acid value of about 4.0.